The instant invention relates to improved enhancement layers for use in electrophotographic imaging processes. The improved enhancement layer of the instant invention is fabricated from semiconductor alloy material, said material characterized by a decreased number of deep midgap defect sites in which charge carriers can be trapped. By moving the Fermi level above midgap and thereby decreasing the number of deep defect sites available to trap charge carriers, the rate of charge carrier emission from those traps is increased and not only is the problem of charge fatigue prevalent in prior art electrophotographic media virtually eliminated, but the problem of image flow is also virtually eliminated.
Electrophotography, also referred to generically as xerography, is an imaging process which relies upon the storage and discharge of an electrostatic charge by a photoconductive material for its operation. A photoconductive material is one which becomes electrically conductive in response to the absorption of illumination; i.e., light incident thereupon generates electron-hole pairs (referred to generally as "charge carriers"), within the bulk of the photoconductive material. It is these charge carriers which provide for the passage of an electrical current through that material for the discharge of the static electrical charge (which charge (either positive or negative) is stored upon the outer surface of the electrophotographic media in the typical electrophotographic process).
First the structure and then the operation of a typical xerographic electrophotographic photoreceptor will be explained so that the operation and advantages of the instant invention may be fully appreciated. It is to be noted, however, that the improved enhancement layer of the instant invention is not limited to use with "typical" photoreceptors, but is equally adapted to be used with any photosensitive material which undergoes a change in any characteristic thereof under the influence of electromagnetic radiation, which characteristic provides for said material to have image reproduction capabilities.
As to the structure: A typical photoreceptor includes a cylindrically-shaped, electrically conductive substrate member, generally formed of a metal such as aluminum. Other substrate configurations, such as planar sheets, curved sheets or metallized flexible belts may likewise be employed. The photoreceptor also includes a photoconductive layer, which, as previously described, is formed of a photoresistive material having a relatively low electrical conductivity in the dark and a relatively high electrical conductivity under illumination. Disposed between the photoconductive layer and the substrate member is a blocking layer, formed either by the oxide naturally occuring on the substrate member, or from a deposited layer of semiconductor alloy material. As will be discussed in greater detail hereinbelow, the blocking layer functions to prevent the flow of unwanted charge carriers from the substrate member into the photoconductive layer. If not for the presence of the blocking layer, charge carriers flowing from the substrate into the photoconductive layer could neutralize the charge stored upon the top surface of the photoreceptor. A typical photoreceptor also generally includes a top protective layer disposed upon the photoconductive layer to stabilize electrostatic charge acceptance against changes due to adsorbed chemical species and to improve the photoreceptor durability. Finally, a photoreceptor also may include an enhancement layer operatively disposed between the photoconductive layer and the top protective layer, the enhancement layer adapted to substantially prevent charge carriers from being caught in deep traps and hence prevent charge fatigue in the photoreceptor.
In operation of the electrophotographic process: the photoreceptor must first be electrostatically charged in the dark. Charging is typically accomplished by a corona discharge or some other such conventional source of static electricity. An image of the object to be photographed, for example a typewritten page, is then projected onto the surface of the charged electrophotographic photoreceptor. Illuminated portions of the photoconductive layer, corresponding to the light areas of the projected image, become electrically conductive and pass the electrostatic charge residing thereupon through to the electrically conductive substrate thereunder, which substrate is generally maintained at ground potential. The unilluminated or weakly illuminated portions of the photoconductive layer remain electrically resistive and therefore continue to be proportionally resistive to the passage of electrical charge to the grounded substrate. Upon termination of the illumination, a latent electrostatic image remains upon the photoreceptor for a finite length of time (the dark decay time period). This latent image is formed by regions of high electrostatic charge (corresponding to dark portions of the projected image) and regions of reduced electrostatic charge (corresponding to light portions of the projected image).
In the next step of the electrophotographic process a fine powdered pigment bearing an appropriate electrostatic charge and generally referred to as a toner, is applied (as by cascading) onto the top surface of the photoreceptor where it adheres to portions thereof which carry the high electrostatic charge. In this manner a pattern is formed upon the top surface of the photoreceptor, said pattern corresponding to the projected image. In a subsequent step the toner is electrostatically attracted and thereby made to adhere to a charged receptor sheet which is typically a sheet of paper or polyester. An image formed of particles of toner material and corresponding to the projected image is thus formed upon the receptor sheet. In order to fix this image, heat and/or pressure is applied while the toner particles remain attracted to the receptor sheet. The foregoing describes a process which is the basis of many commercial systems, such as plain paper copiers and xeroradiographic systems.
It should be clear from the foregoing discussion that in order to obtain high resolution copies, it is desirable that the electrophotographic photoreceptor accept and retain a high static electrical charge in the dark; it must also provide for the flow of the charge carriers which form that charge from portions of the photoreceptor to the grounded substrate, or from the substrate to the charged portions of the photoreceptor under illumination; and it must retain substantially all of the initial charge for an appropriate period of time in the non-illuminated portions without substantial decay thereof. Image-wise discharge of the photoreceptor occurs through the photoconductive process previously described. However, unwanted discharge may occur via charge injection at the top or bottom surface and/or through bulk thermal charge carrier generation in the bulk of the photoconductor material.
A major source of charge injection is at the metal substrate/semiconductor alloy material interface. The metal substrate provides a virtual sea of electrons available for the injection and subsequent neutralization of, for example, the positive static charge on the surface of the photoreceptor. In the absence of any impediment, these electrons would immediately flow into the photoconductive layer; accordingly, all practical electrophotgraphic media include a bottom blocking layer disposed between the substrate and the photoconductive member. This bottom blocking layer is particularly important for electrophotographic devices which employ photoconductors with dark conductivities greater than 10.sup.-13 ohm.sup.-1 cm.sup.-1. As mentioned hereinabove, in some cases the blocking layer may be formed by native oxides occuring upon the surface of the substrate, as for example a layer of alumina occuring on aluminum. In other cases, the blocking layer is formed by chemically treating the surface of the substrate. Since it is particularly important to the electrophotographic copying process to have the semiconductor alloy material exhibit unipolar charging characteristics, an important class of blocking layers is formed by depositing a layer of semiconductor alloy material of appropriate conductivity type onto the substrate to give rise to substantially diode-like blocking conditions.
In order to better understand the manner in which the blocking layers operate, it is necessary to review in greater depth a portion of the physics involved in the blocking layer phenomenon. As previously mentioned, the blocking layer must inhibit the transport and subsequent injection of the appropriate charge carrier (electrons for a positively charged drum) principally from the metal substrate into the body of the photoreceptor. This is accomplished in the doped semiconductor blocking layer by establishing a condition in which the minority charge carrier drift range, mu tau E, is smaller than the blocking layer thickness. Here, mu is the minority carrier mobility, tau is the minority carrier lifetime and E is the electric field strength. One can, for instance, substantially reduce the mu tau product for electrons by doping the blocking layer p-type. The excess holes present in the doped blocking layer greatly increase the probability of electron-hole recombination, thereby reducing the electron lifetime, tau. In effect a condition is achieved whereby electrons injected from the metal substrate recombine with holes in the p-type blocking layer before they are able to drift into the bulk of the photoreceptor to be swept through the top surface and neutralize the static charge thereon. However, while doping can serve to limit the mu tau product for the desired carrier, it can also give rise to deep electronic energy levels in the energy gap of said semiconductor alloy material. This is particularly true for semiconductor alloy material, such as amorphous silicon alloys, in which the efficiency of substitutional doping is not high. These deep levels can become the source of thermally generated carriers or they can, if sufficiently numerous, provide a parallel path for the hopping conduction of electrons through the doped layers. Either of these phonomena can serve to compromise the blocking function of the doped layers.
It should therefore be apparent that amorphous silicon alloys have demonstrated great utility as the material from which to fabricate electrophotographic media as compared to the chalcogenide materials from which such media were previously fabricated. Even the methodology of fabrication has become economical in view of applicants' assignee's use of microwave frequencies in a process disclosed for the first time in commonly assigned U.S. Pat. No. 4,504,518 entitled "Method Of Making Amorphous Semiconductor Alloys And Devices Using Microwave Energy", said process having been specifically adapted for the mass production of electrophotographic media in U.S. Pat. application Ser. No. 580,086 entitled "Method And Apparatus For Making Electrophotographic Devices" (the disclosures of both of these patents being incorporated herein by reference). However, other areas of concern still remain in the fabrication of an electrophotoconductive member capable of high speed, high resolution copying. One area of particular concern resides in the inherent property exhibited by the semiconductor alloy material from which electrophotoconductive layers of prior art constructions were fabricated, e.g., the inherent property of that material to trap charge carriers in deep sites in the energy gap thereof as they reach the interface between the photoconductive layer and the top protective layer. This condition has become known as charge fatigue and occurs when the failure of the charge carriers to quickly vacate traps results in a breakdown of the blocking function of the top protective layer. Once the top protective layer breaks down, a flow of charge carriers is able to freely move therethrough in an attempt to neutralize the electrostatic charge residing on the surface of the electrophotographic medium. This problem, as well as applicants' solution with respect to positively charged media, will be explained in detail in the following paragraphs.
In the course of operation of the typical electrophotographic process, described above, a positive corona charge is placed on the outer surface (the exposed surface of the top protective layer) of the electrophotographic media. The initial reaction of the photoconductive layer of the electrophotographic media to the application of this positive charge to the top surface thereof is to have any free electrons from the bulk be swept toward that surface in an attempt to neutralize the positive charge residing thereon. However, in the movement of these electrons from the bulk of the photoconductive layer to the outer surface of the top protective layer (on which surface the positive charge carriers have accumulated), said electrons encounter deep trap sites such as midgap defect states. While these trap sites are located throughout the bulk of the photoconductive layer, they are of particular importance when they reside near the interface of the photoconductive layer and the top protective layer. This is because the blocking function (the inability of the positive charge carriers electrostatically positioned on the periphery of the top protective layer to penetrate that layer) will cease to be effective (will "breakdown") when an electrical field of sufficient strength is placed across the top protective layer. Obviously, a given density of negative charge carriers trapped near the aforementioned interface of the top protective layer and the photoconductive layer will generate a sufficiently strong electrical field across the top protective layer to cause breakdown, whereas the same number of negative charge carriers trapped in the bulk thereof will not.
Further, trapping sites located deep in the energy gap of a semiconductor alloy material release trapped charge carriers at a much slower rate than do sites located closer to one of the bands. This results from the fact that more thermal energy is required, for example, to re-excite a trapped electron from the deep sites which exist near the middle of the energy gap to the conduction band than is required to re-excite an electron from the shallower sites which exist closer to the conduction band or to reexcite a trapped electron from the deep sites which exist near the middle of the energy gap to the valence band than is required to reexcite an electron from the shallower sites which exist closer to the valence band. The slow release rate from deep traps gives rise to a higher equilibrium trap occupancy and thus a higher electric field distribution.
It is important to note that in the fabrication of the typical electrophotographic photoreceptor which operates with a positive corona charge applied to the outer surface thereof, the photoconductive layer thereof is made from a "pi-type" silicon:fluorine:hydrogen:boron alloy. As used herein, "pi-type" will refer to semiconductor alloy material, the Fermi level of which has been displaced from its undoped position closer to the conduction band to a position approximately "midgap". Further note that as used herein, the term "midgap" will be used to define a point in the energy gap of a semiconductor alloy material which is positioned approximately half-way between the valence band and the conduction band (in the case of 1.8 eV amorphous silicon:fluorine:hydrogen:boron alloy this midgap position is about 0.9 eV from each of the bands). It is necessary to make the photoconductive layer of the photoreceptor pi-type because the typical "intrinsic" amorphous silicon:hydrogen:fluorine alloy as deposited in a glow discharge decomposition process is slightly "nu-type" (the Fermi level of that material is slightly closer to the conduction band than to the valence band) and in a positive corona charge electrophotographic process, the movement of charge carriers through the photoconductive layer under illumination must be maximized while miminizing the thermal generation of charge carriers.
It is to be noted that when the Fermi level is positioned at midgap (as after the addition of the p-dopant to the silicon:fluorine:hydrogen alloy material), electrons moving through a charaged electrophotographic device utilizing said pi-type material will encounter unoccupied deep traps from which they cannot readily emerge. This is because the deepest unoccupied electron trap sites in a layer of semiconductor alloy material lie at or near the Fermi level and in this Pi type material this energy coincides with midgap. The thermal energy required to release an electron from a deep trap is dependent on the depth of that trap. More particularly, the time which a trapped electron will wait, on average, before being thermally emitted from any trap is given by the formula: EQU t=[.nu..sub.0 EXP(-.DELTA.E/kT)]
where ".nu..sub.0 " is the number of times a trapped electron will attempt to escape per second, ".DELTA.E" is the energy required to move an electron from the Fermi lever to the conduction band edge, and kT is the absolute temperature multiplied by Boltzman's constant. ".nu..sub.0 " may be assumed to have a value of approximately 10.sup.12 attempts per second in most solids. For a Fermi level position of 0.9 eV (midgap) the emission time is therefore calculated to be 4.times.103.sup.3 seconds at room temperature. This slow escape time means that it takes approximately 1.2 hours for a electron to vacate the trap. Obviously, an electrophotographic photoreceptor cannot tolerate such a slow electron discharge rate. If, for example, electrons, once trapped, remain confined for such a lengthy period of time, a large concentration of electrons trapped at the photoconductor layer/top protective layer interface will build up with repeated use of the photoreceptor and this space charge and the positive charge accumulated on the surface of the top protective layer will create a very high electric field distortion across said top protective layer, which field causes the top protective layer to "breakdown". As used herein, "breakdown" refers to the inability of the top protective layer to inhibit the flow of charge carriers therethrough.
Applicants have discovered that this breakdown phenomena can be eliminated by reducing the number of defect states which give rise to deep charge carrier traps. As taught in applicant's U.S. Pat. Application Ser. No. 580,081, filed Feb. 14, 1984 and entitled "An Improved Method Of Making A Photoconductive Member And Improved Photoconductive Member Made Thereby", the addition of an "enhancement layer" operatively disposed between the top protective layer and the photoconductive layer beneficially affects the performance of an electrophotographic device incorporating that layer. While at the time of filing said U.S. Pat. No. 580,081 Application, the reason for the physical behavior of the enhancement layer was unknown, Applicants now have determined that the addition of the enhancement layer (as fabricated in the manner taught therein) operated to reduce the escape time of charge carriers caught in deep traps previously encountered at the interface of the photoconductive layer by reducing the overall density of defect states in the semiconductor alloy material from which the enhancement layer was formed. However, the enhancement layer described in the aforementioned copending application, decreased the overall density of defect states by depositing intrinsic semiconductor alloy material by r.f. glow discharge rather than by microwave glow discharge (since microwave deposition tends to create additional defect states). Therefore, the enhancement layer of said aforementioned application relied upon a reduction in the overall density of defect states present in undoped semiconductor alloy material to aid in reducing the number of deep traps in which charge carriers could be caught in order to reduce charge fatigue. However, no attempt or even suggestion of how to optimize the chemical composition of the enhancement layer in order to further prevent charge carriers from being caught in the deep midgap traps was discussed or suggested in said application.
An important advantage obtained by following the teachings of the present invention resides in the optimization of the enhancement layer so as to prevent charge carrier fatigue and improve the operational cycling time of positively charged electrophotographic devices incorporating said optimized enhancement layer. Moreover, by utilizing the disclosure found herein, negative charge carriers (electrons) are substantially inhibited from falling into the deep midgap traps. Only relatively shallow defect states remain in which charge carriers may be trapped and the rate of emission of charge carriers from these shallow traps can be measured in terms of seconds or fractions of a second, rather than in terms of days. Therefore, in its broadest form, the present application relates to the positioning of the Fermi level of the semiconductor alloy material from which the enhancement layer is formed to a position closer to the conduction band than at midgap. This prevented the deep mid-gap states from being occupied by electrons and thus being effective as electron traps. In this way electrons moving through the enhancement layer do not have to pass through a region in which there are effective deep midgap traps. This translates into an electron escape time of less than about 1 second for a 1.8 eV silicon:hydrogen:fluorine:phosphine alloy having the Fermi thereof positioned in the most favored range of 0.75 to 0.65 eV from the conduction band. Because of the quick release time there will be no substantial build up of trapped charge in this region and therefore no high field distortion.
It is noteworthy that the subject inventors do not claim to have invented the concept of fixing the Fermi level of the amorphous semiconductor alloy material from which one of the operative layers of an electrophotographic photoreceptor is fabricated. Rather, said inventors claim to be the first to recognize that it is possible to substantially prevent charge carriers from being caught in deep midgap traps by pinning the Fermi level of the semiconductor alloy material from which the enhancement layer is fabricated at a point approximately 0.8 to 0.5 eV from the conduction band.
Applicants' discovery is to be sharply contrasted to a technique described by Mort, et al in a paper entitled "Field-effect Phenomena in Hydrogenated Amorphous Silicon Photoreceptors" published in the Journal of Applied Physics, Apr. 16, 1984 at page 3197. In this paper, Mort, et al describe a process for the elimination of field effect in photoreceptors, which process was accomplished by the proper doping of the a--Si:H--insulator interface. Mort, et al observed Fermi level motion under the influence of the field generated by corona charging of the electrophotographic photoreceptor, the deleterious effects of which they proposed to counteract by doping. More particularly, Mort, et al proposed the addition of a boron-doped trapping layer interposed between the top surface of the photoconductive layer and the insulating layer (the top protective layer) for quenching the effects of the electric field and removing the effect of "field-induced blurring" (commonly referred to as "image-flow"). In this manner, Mort, et al were able to counteract the problem of "image-flow".
However, Mort, et al were not concerned with and failed to address the concurrently present problem of "charge fatigue". Moreover, Mort, et al, by adding boron dopant, shifted the Fermi level of the semiconductor alloy material toward the valence band for a positively charged media. By so shifting the Fermi level of the semiconductor alloy material, Mort, et al left deep midgap states unoccupied by electrons, which states consequently form deep electron trapping sites. These deep traps are capable of capturing electrons generated either by light exposure or during corona charging, thereby creating the problem of electric field distortion which is repsonsible for the problems of charge fatigue and which the subject application attempts to avoid. Note that Mort, et al specifically prohibit the use of phosphorous doping to shift the Fermi level of the enhancement layer toward the conduction band because such a shift would make the semiconductor alloy material thereof more conductive, thereby causing just the type of lateral electron flow they seek to avoid. Further, there is no disclosure for pinning the Fermi level of the enhancement layer in that shifted position.
In contrast thereto, applicants first intentionally phosphorous doped the semiconductor alloy material of the enhancement layer which is interposed between the photoconductive layer and the top protective layer in order to shift the Fermi level thereof toward the conduction band. By so shifting the Fermi level of the semiconductor alloy material, the electrons do not have to move through and become caught in the deep midgap states present in the energy gap thereof. This substantially eliminates the problems of charge fatigue by keeping the electrons out of the deep midgap states. Applicants then introduce both boron dopant and phosphorus dopant so as to pin the Fermi level at that preselected position in the energy gap through the addition of defect states on both sides of the pinned Fermi level. The added defect states, being shallow, not only solve charge fatigue problems; but those states are sufficiently numerous to pin the Fermi level so as to prevent its field induced movement when the photoreceptor is charged, hence image flow is also inhibited.
As should accordingly be apparent from the foregoing discussion, while Mort, et al propose a solution to the problem of image flow in electrophotographic media, they fail to consider the problem of charge fatigue which their solution to image flow inherently invokes. The subject invention, on the other hand, solves both problems by first appropriately shifting and then pinning the Fermi level of the semiconductor alloy material of a newly added enhancement layer.
In light of the many definitions utilized for the terms "amorphous" and "microcrystalline" in the scientific and patent literature it will be helpful to clarify the definition of those terms as used herein. The term "amorphous", as used herein, is defined to include alloys or materials exhibiting long range disorder, although said alloys or materials may exhibit short or intermediate range order or even contain crystalline inclusions. As used herein the term "microcrystalline" is defined as a unique class of said amorphous materials characterized by a volume fraction of crystalline inclusions, said volume fraction of inclusions being greater than a threshold value at which the onset of substantial changes in certain key parameters such as electrical conductivity, band gap and absorption constant occur. It is to be noted that pursuant to the foregoing definitions, the microcrystalline, materials employed in the practice of the instant invention fall within the generic term "amorphous" as defined hereinabove.
The concept of microcrystalline materials exhibiting a threshold volume fraction of crystalline inclusions at which substantial charges in key parameters occur, can be best understood with reference to the percolation model of disordered materials. Percolation theory, as applied to microcrystalline disordered materials, analogizes properties such as the electrical conductivity manifested by microcrystalline materials, to the percolation of a fluid through a non-homogeneous, semi-permeable medium such as a gravel bed.
Microcrystalline materials are formed of a random network which includes low mobility, highly disordered regions of material surrounding randomized, highly ordered crystalline inclusions or grains having high carrier mobility. Once these crystalline inclusions attain a critical volume fraction of the network, (which critical volume will depend, inter alia, upon the size and/or shape and/or orientation of the inclusions), it becomes a statistical probability that said inclusions are sufficiently interconnected so as to provide a low resistance current path through the network. Therefore at this critical or threshold volume fraction, the material exhibits a sudden increase in conductivity. This analysis (as described in general terms relative to electrical conductivity herein) is well known to those skilled in solid state theory and may be similarly applied to describe additional physical properties of microcrystalline materials, such as optical gap, absorption constant, etc.
The onset of this critical threshold value for the substantial change in physical properties of microcrystalline materials will depend upon the size, shape and orientation of the particular crystalline inclusions, but is relatively constant for different types of materials. It should be noted that while many materials may be broadly classified as "microcrystalline" those materials will not exhibit the properties applicants have found advantageous for the practice of the subject invention unless they have a volume fraction of crystalline inclusions which exceeds the threshold value necessary for substantial change. Accordingly, we have defined "microcrystalline materials" to include only those materials which have reached the threshold value. Further note that the shape of the crystalline inclusions is critical to the volume fraction necessary to reach the threshold value. There exist 1-D, 2-D and 3-D models which predict the volume fraction of inclusions necessary to reach the threshold value, these models being dependent on the shape of the crystalline inclusions. For instance, in a 1-D model (which may be analogized to the flow of charge carriers through a thin wire), the volume fraction of inclusions in the amorphous network must be 100% to reach the threshold value. In the 2-D model (which may be viewed as substantially conically shaped inclusions extending through the thickness of the amorphous network), the volume fraction of inclusions in the amorphous network must be about 45% to reach the threshold value. And finally in the 3-D model (which may be viewed as substantially spherically shaped inclusions in a sea of amorphous material), the volume fraction of inclusions need only be about 16-19% to reach the threshold value. Therefore, amorphous materials (even materials classified as microcrystalline by others in the field) may include crystalline inclusions without being microcrystalline as that term is defined herein.
These and other objects and advantages of the instant invention will be apparent from the detailed description of the invention, the brief description of the drawings and the claims which follow.